The present invention relates to a sealed prismatic battery having a safety vent which opens in an abnormal rise of battery internal pressure.
In recent years, slim and lightweight electronic equipment such as cell phones and personal digital assistances is in widespread use. A small and lightweight battery having high capacity is used as a power supply for the electronic equipment. With increased functionality of the electronic equipment, there is a high demand for battery with higher capacity. A primary battery with use of lithium as an active material and a secondary battery performing charge and discharge operations with use of lithium ion are in particular widely used because of their lightweight and high capacity features, and so the development for the higher capacity is actively being pursued.
In such batteries, upon application of an excessive electric load or application of an excessive thermal load, a short-circuited state is generated inside the batteries, resulting in generation of gas, which leads to an abnormal rise of battery internal pressure. Also in the case where the batteries are put in an over-charged state, gas is generated inside the batteries due to decomposition of electrolyte, which causes an abnormal rise of battery internal pressure. When the battery reaches the point of being unable to withstand the abnormal rise of the internal pressure, it explodes and strews its contents over. Accordingly, the battery has a safety vent which opens upon reaching a specified internal pressure so as to release the battery internal pressure in order to prevent burst of the batteries from occurring.
A sealed prismatic battery is often used as the battery for the electronic equipment and a thinner shape of the sealed prismatic battery is demanded. This leads to decrease of the area of the safety vent, and therefore even when the battery internal pressure reaches a specified value, it becomes difficult to open the safety vent only with the internal pressure of the battery to release the internal pressure as shown in JP 2001-256944 A (pages 2 to 4, FIGS. 1 to 12) and JP H11-273640 A (pages 3 to 5, FIGS. 4 to 8). If the safety vent provided on a cap is enlarged or the thickness of a thin film section or a thin-walled section is decreased in order to enhance opening efficiency, there is a possibility of not only the damage on the vent and the deformation of the cap upon impact caused by falling of the battery or the like but also the opening of the vent when the battery is stored in the normal circumstance.
Accordingly, as shown in JP H10-261391 A (pages 2 to 3, FIGS. 2 to 4), JP 2000-260410 A (pages 2 to 3, FIGS. 1 and 2), and JP 2002-008615 A (pages 2 to 4, FIGS. 1 and 2), there has been used a vent structure in which a vent opens with relatively low internal pressure of the battery with use of deformation due to expansion of the battery caused by a rise of the internal pressure thereof, although the structure does not fully satisfy the demand for enhancing impact resistance.